Production of alpha-lipoic acid intermediates



Umtfid States ?lete t PRODUCTION OF a-LIPUIC ACID INTERMEDIATES Edward Walton, Scotch Plains, N. 3., assignor to Merck & Co., Inc., Rahway, N. 3., a corporation of New Jersey No Drawing. Application July 22, 1954, Serial No. 445,165

20 Claims. (Cl. 260-455) This invention relates to the production of .a-lipoic acid. More particularly, this invention is concerned with novel processes of producing dithiolalkanoic acids and salts thereof of the formula CH1OH2CH(OH2)GO OH SH SH XI and XII wherein n is an integer from 1 to 8 and novel intermediate compounds useful in such processes. It is also concerned with novel methods of producing and pure enantiomorphic forms of a-lipoic acid.

Compounds of the above formula are valuable in the production of other chemical compounds. Thus, compounds in which n is 4, i. e. 6,8-dithioloctanoic acid, are particularly useful precursors in the synthesis of a-lipoic acid or 5-[3-(1,2-dithiacyclopentyl)1 pentanoic acid. In this regard, 5-[3-(l,2-dithiacyclopentyl)] pentanoic acid is a valuable growth stimulating substance which wasobtained from liver and later reported to have the structure as is disclosed in the I. Am. Chem. Soc. 74, 3455 (1952.).

Production of the dithiolalkanoic acids is achieved by a novel sequence of reactions which shall be illustrated hereafter by specific methods of producing 6,S,-dithioloctanoio acid and its pure enantiomorphic forms. These reactions, however, are not to be considered restricted to production of this compound since they are equally applicable to the production of the corresponding analogs in which 11 may be 1 or an integer up to and including '8.

The production of 6,8-dithioloctanoic acid according to the subject invention may be conveniently considered in terms of two related phases. In Phase A, a 7-monoester of 7-carboXy-2-heptenoic acid (I) is converted by novel processes to 6-thiol-S-hydroxyoctanoic acid (VIII). In Phase B, 6-thiol-S-hydroxyoctanoic acid, the 6-acylthio precursors hereto and esters of such compounds are converted to 6,8-dithioloctanoic acid.

The first sequence of reactions in Phase A comprises reacting a 7-monoester of 7-carboXy-2-heptenoic acid (I) with a thiocarboxylic acid to produce the corresponding 7-monoester of a 3-acylthio-7-carboxyheptanoic acid (H), halogenating said compound to produce the corresponding heptanoyl halide (HI), and reducing said compound with an alkali metal borohydride to form the corresponding ester of the 6-acylthio-S-hydroxyoctanoic acid '(IV). The ester of 6-acylthio-8-hydroxyoctanoic acid (IV) may then be converted to 6-thiol-S-hydroxyoctanoic acid according to one method by treating said compound with a base to form directly the corresponding ester of 6- thiol-8-hydroxyoctanoic acid (VI), treating said ester with sufiicient base to form the corresponding di-metal salt (VII) and treating said salt with acid to form the desired 6-thiol-8-hydroxyoctanoic acid (VIII). In'avariation of this process the ester of 6-acylthio-8-hydroxyoc tanoic acid (IV) may be treated with suflicient base to form a mono-salt of the 6-thio1-8-hydroxyoctanoate (V) v 2,792,414 Patented May 14, 1957 wherein R is a hydrocarbon group such as an alkyl, alkenyl, aryl or aralkyl group, and particularly such groups having 1 through 8 carbons. R is an acyl group such as an alkylacyl, alkenylacyl, arylacyl or aralkacyl group, and particularly such acyl groups derived from carboxylic acids and having 1 through 8 carbons, X is an anion of a strong acid and Y is an alkali metal or alkaline earth metal.

The production of 6,8-dithioloctanoic acid is achieved according to Phase B by reacting 6-thiol-8-hydroxyoctanoic acid, 6-acylthio precursors thereof or esters thereof, produced as indicated above, with thiourea or an N-su'bstituted thiourea in the presence of a strong acid to form 6-thiol-8-[2 (2-thiopseudoureido)loctanoic acid or the corresponding N-substituted thiopseudoureide as an acid addition salt IX, neutralizing said salt to form the free base X, hydrolyzing said compound with a baseto form the corresponding tri-alkali metal salt of 6,8-dithioloctanoic acid XI and treating said salt with acid to form 6,8-dithioloctanoic acid. This process may be conven iently represented as follows:

0 R 0 3 (CHghCHOH CHz'OH Sli.

(IV), (VI) 01 (VIII) Phase" B wherein R and R are hydrogen or hydrocarbon groups such as alkyl, alkenyl, aryl and aralkyl groups, particularly such groups having 1 through 8 carbons. R is hydrogen or an alkylacyl, alkenylacyl, arylacyl or aralkylacyl group, particularly such groups derived from carboxylic acids having 1 through 8 carbons, and Y is an alkali metal or alkaline earth metal, and X is. an anion of a strong acid.

The starting materials which are used in this-invention are readily prepared by methods known in the art. Thus, 7-monoesters of 7-carboxy-2-heptenoic acid and similar homologs may be prepared as disclosed in the J. Org.

;Chem.i12, 160 (1947).- Examples of such compounds which may; be prepared are 7-carbomethoxy-Z-heptenoic acid, 7-carboethoxy-2-heptenoic acid, 7-carbopropoxy-2- heptenoic acid, 7-carbophenoxy-2-heptenoic acid, 7-carbobenzyloxy-Z-heptenoic acid and homologs thereof.

The first reaction in this process is effected by contactmg a 7-monoestcr of 7-carboxy-2-heptenoic acid with a thiocarboxylic acid to produce the corresponding 7-monoester of the 3-acylthio-7-carboxy heptanoic acid. Thiocarboxylic acids such as thioacetic acid, thiopropionic acid, thiobutyric acid, thiovaleric acid, thiocaproic acid,

i'thiobenzoic acid, and thiophenylaceti'c acid are some examples of compounds which are useful reactants in this step. The reaction proceeds in the presence of an inert solvent such as benzene, chloroform, carbon tetrajchloride, toluene, dioxane and ethanol. The reaction can be effected at room temperature or elevated temperatures 'up to'about'7 0 C. It is completed in about 4 to 12 hours depending on the reaction temperature. The product is recovered from the reaction mixture by ordinary procedures such as by extraction into a water-immiscible solvent, washing the organic extract with water and removing the solvent under reduced pressure.

In a spec1fic illustration of this reaction, thioacetic acid is reacted with 7-carboethoxy-2-heptenoic acid to produce 3-acetylthio-7-carboethoxyheptanoic acid. Other compounds produced in the described manner from the appropriate reactants that might be mentioned are 3-propionylthio-7-carbomethoxy heptanoic acid, 3-butyrylthio-7-carbophenoxyheptanoic acid, 3-benZoylthio-7-carbopropoxyheptanoic acid, 3-phenylacetylthio-7-carboethoxyheptanoic acid, and the like.

In the United States patent application of Walton and Wagner, Serial No. 445,166, filed July 22, 1954, there is disclosed a method of resolving 3-acetylthio-7-carboethoxyheptanoic acid into its and enantiomorphic forms. Such resolved forms of this compound, as well as its racemic form may be employed in the following reactions and accordingly the resulting products produced therefrom will be the appropriate enantiomorphic forms.

The 7-monoester of the-3-acylthio-7-carboxyheptanoic acid prepared above is then reacted with a halogenating agent to produce the corresponding 7-monoester of the 3-acylthio-7-carboxyheptanoyl halide. This may be achieved by contacting the heptanoic acid with halogenating agents such as thionyl chloride, phosphorous oxychloride, phosphorous trichloride or tribromide and phosphorous pentabromide or pentachloride. The reaction is conveniently achieved under solvent conditions attained either by use of an excess of the halogenating agent or by the use of an inert solvent such as benzene, ether, dioxane and the like. Room temperature or slightly elevated temperatures may be employed for the reaction. Following completion of the reaction, the desired 7-monoester of the 3-acylthio-7-carboxyheptanoyl halide may be recovered from the reaction mixture by removal of the excess halogenating agent and/or solvent. The usual procedures procedures of extraction and evaporation can be used for this purpose.

Illustrative of the novel compounds which are produced in this manner are 3-acetylthio-7-carboethoxy heptanoyl chloride and the and forms thereof by the reaction of 3-acetylthio-7-carboethoxyheptanoic acid and the and forms thereof with thionyl chloride or another suitable chlorinating agent. The corresponding bromides are also produced in this way. Similarly, compounds such as 3-propionylthio-7-carbomethoxyheptanoyl chloride and 3-benzoylthio-7-carbophenoxyheptanoyl bromide are produced from the appropriate heptanoic acids.

The 7-monoester of the 3-acylthio-7-carboxyheptanoyl halide produced as above is subsequently treated with an alkali metal borohydride to efiect reduction of the heptanoyl halide to the corresponding heptanol or ester of the 6-acylthio-8-hydroxyoctanoate. Alkali metal borolhydrides such as sodium, potassium and lithium borohycompose the excess alkali metal borohydride, followed by the addition of acid until the mixture is neutral or slightly acidic. The principal product obtained from the reaction is the corresponding ester of the 6-acylthio8- hydroxyoctanoic acid. However, small amounts of side products are also produced as a result of the formation of an alkali metal hydroxide during the reaction which effects partial hydrolysis of the acyl groups before the reduction is complete. Thus, products such as the corresponding ester of 6-mercapto-8-hydroxy octanoic acid and 6-mercapto-8-hydroxy octanoic acid itself are also formed in small amounts during the reaction. The presence of these compounds is not detrimental to the process, however, because in the following steps all of these compounds are converted to the desired end product. It is accordingly unnecessary to separate such products before continuing with the next reaction.

According to a specific illustration of the described reduction 3-acetylthio-7-carbocthoxyheptanoyl chloride is reacted with sodium borohydride to form ethyl G-acetylthio-8-hydroxyoctanoate. This reaction may be repeated with the and enantiomorphic forms of the starting material to produce said product in an optically active form. v In a like manner, using the appropriate fprecursor, there are obtained compounds such as methyl 6-propionylthio-S-hydroxyoctanoate and phenyl o-benzoylthio-S-hydroxyoctanoate. u Conversion of the esters of 6acylthio-8-hydroxyoctanoic acid (IV) to esters of 6thiol-8-hydroxyoctanoic acid and to metal salts thereof is conveniently achieved by treatment with a base. By employing suitable equivalent amounts of base the hydrolysis may be effected so as to produce completely or partially saponified products. Thus, use of three equivalents of base gives a dimetal salt of 6-thiol-8-hydroxyoctanoic acid (VII) which when treated withacid isconverted to 6-thiol-8-hydroxyoctanoic acid.

Alternatively, treatment of the ester (IV) with approximately one equivalent of base will aiford the corresponding carboxylic acid ester of 6-thiol 8-hydroxyoctanoic acid (W). This latter compound may then be treated with additional base, one equivalent'leading to the monometal salt of the 6-thiol-8-hydroxyoctanoic acid ester (V), and two equivalents giving a di-metal salt of the free octanoic acid.

Alkali metal or alkaline earth .metal hydroxides or carbonates are the preferred bases for efiecting the saponifications discussed above. The reactions are conveniently accomplished by intimately .contacting the base and octanoic acid derivative in an aqueous organic sol- .vent such as aqueous methanol or ethanol. Warming the reaction mixture serves to promote the hydrolysis. The end product may be recovered from the reaction mixture by conventional methods.

The metal salts obtained by treatment of a 6-acylthio- 8-hydroxyoctanoic acid ester with base-are converted to .6-thiol-8-hydroxyoctanoic acid by treatment with acid.

Referring .now to Phase B, the .first reaction in this process comprises contacting 6-tbiol-8-hydroxyoctanoic acid, 6-acylthio precursors thereof and esters thereof With thiourea or an N-substituted thiourea to form -6-thiol-8- [2-(2-thiopseudoureido)] octanoic acid or the corresponding N-substituted thiopseudoureide. Examples of starting materials of the type which may be used in this reaction are those compounds previously disclosed here- 'inabove such as 6-thio-8 hydroxyoctanoic acid and esters thereof such 'as the methyl, ethyl, propyl, butyl, phenyl, benzyl'and phenylethyl and 6-acylthio precursors thereof such as ethyl 6-acetylthio-8-hydroxyoctanoate, methyl 6- propionylthio-S-hydroxyoctanoate and phenyl fi-benzoyltbio-8 hydroxyoctanoate. In addition to thiourea, N-hydrocarbon substituted thioureas such as .N-methy'lthiourea, N-ethylthiourea, N-phcnylthiourea, and N-benzylthiourea may be employed inthe reaction. The desired reaction is conveniently achieved by contacting the reactants in the presence of a strong acid such as a mineral acid or Lewis acid in aqueous anhydrous solution at room temperature'or elevated temperatures, preferably at the reflux temperature. Hydrochloric acid, hydriodic acid, hydrobromic acid, sulfuric acid, boron trifluor-ide and aluminum chloride are examples-of acids suitable for use in this reaction. The reaction is completed quickly and thedesiredproduct'may be conveniently isolated in the form of an acid addition salt by conventional methods.

According to'thedescribed method, 6-thiol-8-hydroxyoctanoic acid is reacted with thiourea in *the presence of hydrogen bromide to produce 6-thiol-8-I2-(2-thiopseudo ureido),] octanoic acid hydrobromide. By replacing hydrobromic acid with a difierent acidother acid addition salts thereof may be produced. In a similar manner, but employing the appropriate N-substituted thiourea there are obtained acid addition salts of 6-thiol-8-[2-(l-propyl- 2-'thiopseudoureido)] octanoic acid, 6-thiol-8-I2-(l-benzyl-2-thi0pseudoureido)] octanoic acid and 6-thiol-.8-.[2- (1-phenyl-2-thiopseudoureido)l octanoic acid. By neutralizing the acid addition salts of 6-thiol 8-[2-(2-thiopseudoureidofl octanoic acid and the N-substituted thiopseudoureide compounds the corresponding free bases may-be produced and isolated by known techniques.

A 6,8-dithioloctanoic acid tri-metal salt is subsequently produced either by treating a 6-thiol-8-[2-(2-thiopseudoureido')] octanoic acid or similar N-substituted thiopseudoureide compound in the form of the free base or as an acid addition salt thereof, with a suitablebase in the reflux temperature.

After the reaction a has been completed the resulting-salt may be recovered if desired. Preferably, however, the reaction mixtureis treated directly With acid toconvert such salts to 6,8-dithioloctanoic acid.

By treating .6,8-dithioloctanoic acid with a mild oxidiz- .ing agent such as iodine-potassium iodide, a-lipoic acid-is formed. The (+)-a-lipoic acid and ()-oz.liPOiC acid forms are similarly produced by oxidation of the optically active forms of 6,8dithioloctanoic-acid.

The following examples are included to illustrate specific embodiments of the invention. It is understood, however, that the invention is not to be limited to the specific disclosure of these examples.

EXAMPLE 1 3-acetylthi0-7-carb0eth0xyheplanoic acid i-butyrylthio-7-carbophenoxyheptanoic acid To ml. of benzene is added 8 g. of tbiobutyric acid and 17 g. of 7-carbophenoxy-Z-heptenoic acid. After standing at room temperature overnight the mixture is diluted with cold water and thesolvents removed under reduced pressure to yield 3-butyrylthio-7-carbophenoxyheptanoic acid.

Similarly, by reacting other thiocarboxylic acids and the appropriate 7-monoester of Z-heptenoic acid there are produced other novel compounds such as 3-propionylthio-7-carbomethoxyheptanoic acid, '3-benzoylthio-7-carbopropoxyheptanoic acid and '3-phenylacetylthi0-7-carboethoxyheptanoic acid.

EXAMPLE 3 -3- acetyZthi0-7-carboethoxyheptanoyl chloride A 20.6 g. sample of (+)-3-acetylthio-7-carboethoxyheptanoic acid is treated with 13.3 g. of thionyl chloride and held atroom temperature overnight. Thereaction mixture is worked up as in Example 5 to isolate 21.5 g. of '(+)-3-acetylthio-7-carboethoxyheptanoyl chloride; [ocJ +49 (c., 1.42; benzene).

EXAMPLE 4 -3-acetylthio-7-carboethoxyheptanoyl chloride A 22.8 g. sample of ()-3acetylthio-7-carboethoxyheptanoic acid is treated with 14.8 g. of thionyl chloride. The mixture is kept at room temperature overnight and Worked up as in Example 5 to yield 24.3 g. of ()-3- acetylthio-7-carboethoxyheptanoyl chloride; [a1 36 (c., 1.4, benzene).

EXAMPLE 5 3-acetylthio-7-carboethoxyhepranoyl chloride Approximately 6.4 g. of thionyl chloride is added to 10 g. of 3-acetylthio-7-carboethoxyheptanoic acid. The solution is kept at room temperature overnight and the. excess thionyl chloride is removed by concentration under reduced pressure. Benzene is added to the residue and the benzene solution evaporated to dryness under reduced pressure; this is repeated and finally the 3-acetylthio-T-carboethoxyheptanoyl chloride is held under reduced pressure overnight to remove residual volatiles;

neutralization equivalent calcd for C12H1904SC1 147; found, 145.

'EXAMPLE6 3-bzdyrylthi0-7-carbophenoxyheptanoyl chloride Ethyl 6-acetylthio-8-hydr0xy0ctanoate To a suspension of 5.8 g. of sodium borohydride in 70 ml. of dioxane is added a solution of 8.5 g. of 3- acetylthio 7 carboethoxyheptanoyl chloride in 30 ml. of dioxane with stirring over a minute period. An additional 2 g. of sodium borohydride is added and stirring is continued for 2 hours at 25 C. The reaction mixture is cooled to less than 10 C. and ml. of water is added as rapidly as possible. Cold aqueous hydrochloric acid is then added to about pH 2. The reaction mixture is diluted to 300 ml. with water and extracted with chloroform. After washing with aqueous potassium bicarbonate and water, the chloroform extract is dried and concentrated under reduced pressure to yield ethyl 6-acetylthio-8-hydroxyoctanoate in which is mixed some ethyl 6-thiol-S-hydroxyoctanoate; n 1.49.

EXAMPLE 8 I Ethyl 6-acetylthi0-8-hydroxyoctan0ate derived from 3-acetylthio-7-carboethoxyheptan0yl ChI ride Approximately 19 g. of sodium borohydride is suspended in 200 ml. of dioxane and 23.6 g. of ()-3- acetylthio-7-carboethoxyheptanoyl chloride dissolved in 50 ml. of dioxane is added. The mixture is Worked up in a manner similar to Example 7 to yield 15.9 g. of ethyl 6-acetylthio-8-hydroxy octanoate, B. P. 115-125" C./0.1 mm., derived from ()-3-acetylthio-7-carboxyheptanoyl chloride; [al 3 +l0.3 (c., 1.45 methanol).

EXAMPLE 9 Ethyl 6-acetylthio-8-hydr0xy0ctan0ate derived from 3-acetylthi0-7-carb0eth0xyheptan0yl chloride 19 grams of sodium borohydride is suspended in 200 ml. of dioxane and 21 g. of (.'+)-3-acetylthio-7-carboethoxyheptanoyl chloride in 50 ml. of dioxane is added. The mixture is worked up as in Example 7 to yield 8.3 g. of ethyl 6-acetylthio-s-hydroxyoctanoate derived from -3-acetylthio-7-carboethoxyheptanoyl chloride; [041 2 (c., 1.93; methanol).

EXAMPLE 10 Ethyl 6 -butyrylthio-S-hydroxyoctanoate EXAMPLE 11 Ethyl 6-thiol-8-hydroxyoctanoate A 4 g. sample of ethyl 6-acetylthio-S-hydroxyoctanoate, produced as in Example 7, is dissolved in ethanol with a few drops of phenolphthalein solution and dilute sodium hydroxide is added slowly while the solution is warmed. The dilute sodium hydroxide is added until it is no longer consumed rapidly, as evidenced by an increase in pH of the mixture. The mixture is .cooledand acidified to pH 3. The ethanol is removed by. evaporation under reduced pressure and the residual aqueous phase is ex- :tracted with chloroform. The chloroform solution is dried and concentrated to yield ethyl 6-thiol-8-hydroxyoctanoate.

This procedure is also followed to produce the appropriate optical isomers of ethyl 6-thiol-8-hydroxyoctanoate derived from and 3-acetylthio-7-carboethoxyheptanoyl chloride. V

Further treatment of the ethyl esters with a base and then acid gives the corresponding racemate and the and enantiomorphs of 6-thiol-8-hydroxyoctanoic acid.

EXAMPLE 12 Ethyl 6-thiol-8-hydr0xy0ctan0at About 3 g. of ethyl 6-butyrylthio-8-hydroxyoctanoate is added to 30 ml. of alcohol. The solution is warmed slowly as dilute sodium hydroxide is added until the butyryl group is hydrolyzed. This is effected when the pH shifts upon the addition of the sodium hydroxide. The alcohol is evaporated and ether added to the aqueous residue. The organic phase is separated, dried and evaporated to yield ethyl 6-thiol-8-hydroxy octanoate.

This procedure is followed to produce other esters of 6-thiol-8-hydroxyoctanoic acid from the corresponding 6- acylthio compounds. Examples of other esters which are 50 produced are methyl 6-thiol-8-hydroxyoctanoate, propyl 6-thiol-8-hydroxyoctanoate, benzyl 6-thiol-8-hydroxyoctanoate and phenyl 6-thiol-8-hydroxyoctanoate. Such compounds may be treated furtherwith a base like sodium hydroxide followed by treatment with acid to convert such compounds to 6-thiol-8-hydroxyoctanoic acid.

EXAMPLE 13 6-thiol-8-hydr0xy0ctan oic acid and its di-sodium salt A 2.4 g. sample of ethyl 6-acetylthio-8-hydroxyoctanoate formed in Example 7 is added to a solution of 4 ml. of 30% aqueous sodium hydroxide in 10 ml. of methanol. A trace of zinc dust is added to maintain a reducing atmosphere and the mixture is refluxed for 1 hour. The solution containing the disodium salt of 6- thiol-8-hydroxyoctanoic acid is acidified with hydrochloric acid and extracted with chloroform. The chloroform extract is washed with aqueous potassium bicarbonate and the aqueous extract is acidified. It is extracted with chloroform and the chloroform is evaporated under reduced pressure to give 6-thiol-8-hydroxyoctanoic acid: neutralization equivalent; calcd for CaH1sOaS 192; found, 193; n =1.499.

EXAMPLE 1 4 6-thiol-8-hydroxyoctanoic acid and its iii-sodium salt derived from (+)-3-acetylthi0-7-carb0eth0xyheptan0yl chloride A 8.2 g. sample of the product obtained in Example 9 is added to a solution of 14 ml. of 30% aqueous sodium hydroxide in 30 ml. of methanol. A trace of zinc dust is added and the mixture is refluxed for 1 hour. The solution containing the appropriate optical isomer of 6-thiol-8- hydroxyoctanoic acid di-sodium salt is treated as in Example 13 to recover the optical isomer as the free acid.

EXAMPLE 15 6 thi0l-8-hydroxyoctan0ic acid and its di-sodium salt derived from (-)-3-acetylthz'0-7-carboethoxyheptanoyl chloride A 16.8 g. sample of ethyl 6-acetylthio-8-hydl'oxy-v octanoate produced as in Example 8 is added to a solu:

wan;

tion of 28.rnl. of 30% aqueous sodium hydroxide in 60 ml. of methanol. A trace of, zinc dust is added andthe mixture is refluxed for 1 hour. The solution of 6thiol-8- hydroxyoctanoic acid di-sodium salt having the appropriate optical activity is then treated as in Example 13 to recover the optically active free acid.

EXAMPLE 16 6-thi0l-8-[2-(Z-thiopseudoureido)l octanoic acid and 6,8-

dz'thioloctanoic acid and its tri-sodium. salt A7.9 g. sample of 6-thiol-8-hydroxyoctanoic acid produced as in Example 13 .is refluxed in 50 ml. of 40% hydrobromic acid with 9.4 g. of thiourea for 7 hours. The reaction mixture containing 6-thiol-8-[2-(2-thiopseudoureido)] octanoic acid hydrobromide is neutralized with sodium hydroxide to yield a solution of 6-thiol-8- [2-(2-thiopseudoureido)] octanoic acid. Sodium hydrox ide is added until the mixture is alkaline. It is refluxed to form the trisodium salt of 6,8-dithioloctanoic acid. The mixture is acidified with hydrochloric acid to form 6,8- dithioloctanoic acid which is extracted with chloroform. The product is isolated by evaporation of the chloroform.

EXAMPLE 17 6-thi0l-8-[2-(2-thi0pseadoureid0)] octanoic acid and 6,8 dithioloctanoic acid and its tri-sodium salt derived from -3-acetylthi0-7-carb0ethoxyheptanoyl chloride A 5 g. sample of 6-thiol-8-hydroxyoctanoic acid (derived from ()-3-acetylthio 7 carboethoxyheptanoyl chloride and produced as in Example is treated with thiourea and hydrobromic acid as in Example 16. The reaction mixture containing the related enantiomorphic form of 6-thiol-8-[2-(2-thiopseudoureido)] octanoic acid hydrobromide is worked up as in Example 16 to yield the corresponding enantiomorphic forms of 6-thiol-8-[2-(2- thiopseudoureido)] octanoic acid, tri-sodium salt of 6,8- dithioloctanoic acid and 6,8-dithioloctanoic acid.

EXAMPLE 18 6-tlziol-8-[2-(2-thi0pseudoureid0)l octanoic acid and 6,8-

dithioloctanoic acid and its tri-sodium salt derived from -3-acetylthio-7-carb0ethoxyheptanoyl chloride A 5 g. portion of 6-thiol-8-hydroxyoctanoic acid (derived from (+)-3-acetylthio 7 carboethoxyheptanoyl chloride and produced as in Example 14) is treated with thiourea and hydrobromic acid as in Example 16. The reaction mixture containing the related enantiomorphic form of 6-thiol-8-[2-(2-thiopseudoureido)] octanoic acid hydrobromide is Worked up as above to yield the corresponding enantiomorphic forms of 6-thiol-8-[2-(2-thiopseudoureido)] octanoic acid, tri-sodium salt of 6,8-dithioloctanoic acid and 6,8-dithioloctanoic acid.

EXAMPLE l9 (-)-a-lip0ic acid The 6,8-dithioloctanoic acid produced in Example 17 is dissolved in chloroform and treated with an aqueous solution of iodine-potassium iodide until the iodine color persists. The chloroform layer is washed with a dilute aqueous solution of sodium bisulfite to remove excess iodine. It is then dried and evaporated to yield a residue of (-)-a-lipoic acid. The product is extracted with warm cyclohexane which on being cooled yields yellow crystals of essentially pure ('--)-C-1lPOiC acid.

EXAMPLE -a-lip0ic acid The 6,8-dithioloctanoic acid produced in Example 18 is added to chloroform and oxidized with iodine-potassium iodide as in Example 19 to produce )-a-lipoic acid which is isolated as pure yellow crystals.

. 10. EXAMPLE 21' H a 6-thiol-8-[2-(2-flziopseudoursidol] octanoic acidand 6,8-dithf0l0ctanoic acid and its tri-sodilim salt A 2 g. portion of ethyl 6-acetylthio-8-hydroxy octanoate and ethyl 6-thiol-8-hydroxy octanoate produced in Example 7 is refluxed with 1.5 g. of thiourea and 10 ml. of 40% hydrobromic acid for 24 hours. The reaction mixture containing 6-tl1iol-8-[2-(2-thiopseudoureido)l octanoic acid hydrobromide is neutralized with sodium hydroxide to yield a solution of 6-thiol-8-[2-(2-thiopseudoureido) Joctanoic acid. Sodium hydroxide is added until the mixture is alkaline and then the mixture is re fluxed, thereby forming the tri-sodium salt of 6,8-dithioloctanoic acid. The mixture is acidified with hydrochloric acid to form 6,8-dithioloctanoicacid which is extracted into chloroform. The product is isolated byevaporation of the chloroform. Various changes and modifications of the invention can be made and, to the extent that such variations incorporate the spirit of this invention, they are intended to be ineluded within the scope of the appended claims.

What is claimed is:

1. The process which comprises reacting a 7-alkyl monoester of a 7-carboxy-2-heptenoic acid wherein the alkyl group contains from one to eight carbon atoms with a lower aliphatic thiocarboxylic acid to produce the corresponding 7-alkyl monoester of a 3-lower aliphatic acylthio-7-carboxyheptanoic acid.

2. The process which comprises reacting 7-carboethoxy- Z-heptenoic acid with thioacetic acid to produce 3- acety'lthio-7-carboethoxyheptanoic acid.

3. The process which comprises reacting a 7-alkyl monoester of a 3-lower aliphatic acylthio-7-carboxyheptanoic acid wherein the alkyl group contains from one to eight carbon atoms with a halogenating agent to produce the corresponding 7-alkyl monoester of a 3-lower aliphatic acylthio-7-carboxyheptanoyl halide.

4. The process which comprises reacting 3-acetylthio- 7-carboethoxyl1eptanoic acid with a halogenating agent to produce the corresponding 3-acetylthio-7-carboethoxyheptanoyl halide.

5. The process which comprises reacting 3-acetylthio-7- carboethoxyheptanoic acid with a chlorinating agent to produce 3-acetylthio-7-carboethoxyheptanoyl chloride.

6. The process which comprises reacting a 7-alkyl monoester of a 7-carboxy-2-heptenoic acid wherein the alkyl group contains from one to eight carbon atoms with a lower aliphatic thiocarboxylic acid and halogenating the resulting compound to produce the corresponding 7-alkyl monoester of a 3-lower aliphatic acylthio-7-carboxyheptanoyl halide.

7. A 7-alkyl monoester of a 3-lower aliphatic carboxylic acylthio7-carboxyheptanoic acid wherein the alkyl group contains from one to eight carbon atoms.

8. A 7-alkyl monoester of a 3-lower aliphatic carboxylic acylthio-7-carboxyheptanoyl halide wherein the alkyl group contains from one to eight carbon atoms.

9. 3-acetylthio-7carboethoxyheptanoic acid.

10. 3-acetylthio-7-carboethoxyheptanoyl chloride.

11. -3-acetylthio-7-carboethoxyheptanoyl chloride.

12. ()-3-acetylthio-7-carboethoxyheptanoyl chloride.

13. 3-butyrylthio-7-carbophenoxyheptanoic acid.

14. 3-butyrylthio-7-carbophenoxyheptanoyl chloride.

15. The process which comprises reacting a 7-alkyl monoester of a 3-lower aliphatic carboxylic acylthio-7- carboxyheptanoyl halide wherein the alkyl group contains from one to eight carbon atoms with an alkali metal borohydride and treating the reaction mixture with acid until it is no longer basic to produce the corresponding alkyl ester of a 6-lower aliphatic carboxylic acylthio-S- hydroxyoctanoic acid.

16. The process which comprises reacting 3 acetylthio- 7-carboethoxyheptanoyl chloride with sodium borohydride and treating the reaction mixture with acid until it 1 1 is no longer basic to produce ethyl 6-acetylthio'8-hydroxyoctanoate.

, 17. The process which comprises reacting a compound of the formula wherein R is a member of the group consisting of hydrogen and hydrocarbon groups containing from one to eight carbon atoms and R is a member of the group consisting of hydrogen and acyl radical of an aliphatic carboxylic acid containing from one to eight carbon atoms, with a compound of the formula HaN-( JNHR wherein R is a member of the group consisting of hydrogen and hydrocarbon groups containing from one to eight carbon atoms, in the presence of a strong acid to form the corresponding acid addition salt of a compound of the formula wherein R has the significance previously assigned.

thio-8-hydroxyoctanoate with thiourea in the presence of a strong acid to produce the corresponding acid addition salt of 6-thiol-8-[2-(2-thiopseudoureido)] ,octanoic acid.

References Cited in the file of this patent UNITED STATES PATENTS Jayne et al July 31, 1945 Hawley Mar. 24, 1953 OTHER REFERENCES Bullock: 74 I. A. C. S., 3455. 

9. 3-ACETYLTHIO-7-CARBOETHOXYHEPTANOIC ACID. 